1. Field the the Invention
The present invention relates to a control device for a hydraulically driven cooling fan of an engine in a vehicle such as an automobile.
2. Description of the Prior Art
For an internal combustion engine in a vehicle such as an automobile, it has already been thought of to drive a cooling fan for supplying cooling draft to the radiator of the engine by a hydraulically driven motor such as an oil-hydraulic motor, and to control the rotation rate of the motor according to an engine temperature parameter such as engine coolant temperature, as disclosed in, for example, Japanese Utility Model Publication Sho 49-40183 (1974) and Japanese Patent Laying Open Publication Sho 58-13119 (1983).
In such a hydraulically driven cooling fan the rotation rate of the fan is controlled by the flow rate of the hydraulic operating fluid supplied to the hydraulic motor.
As a control device for controlling the rotation rate of such a hydraulically driven cooling fan it has been thought of to provide a flow constriction means in a passage for conducting the hydraulic operating fluid from a pump to the hydraulic motor, a first pressure chamber on an upstream side of the flow constriction means along the flow direction of the fluid from the pump to the motor, a second pressure chamber on a downstream side of the flow constriction means opposite to said upstream side, and a pressure control means responsive to the pressure difference between the first and the second pressure chamber so as to leak selectively part of the fluid from the upstream side of the flow constriction means when the pressure difference increases beyond a preset value so as thereby to maintain a constant pressure difference across the flow constriction means, wherein the cross sectional opening area through the flow constriction means is varied by a variable lift control means such as a linear solenoid valve according to the requirement for the draft of the fan.
Such a control device for the hydraulically driven fan operates satisfactorily to control the rotation rate of the fan under normal operating condition of the vehicle. However, when the engine is started at cold state, the pressure of the fluid supplied to the pump can temporarily rise to an abnormally high pressure due to a delay in selectively leaking part of the operating fluid from the upstream side of the flow constriction means caused by high viscosity of the operating fluid at low temperature. Particularly when a smoothing choke means is provided in the passage transmitting the fluid pressure from the downstream of the flow constriction means to the second pressure chamber to prevent hunting of the control device, the choke means presents high flow resistance against the transmittance of the pressure to the second pressure chamber, and therefore the pressure control operation by the pressure control means is further delayed. Such abnormal rise of the hydraulic pressure deteriorates the fuel consumption of the vehicle and the durability of the hydraulic motor.
When the pump has a built-in relief valve, such abnormal rise of the hydraulic pressure can be avoided. However, when the cooling fan is subject to a high driving draft during high speed driving, the fan rotates by itself at a high rate, whereupon the hydraulic motor for driving the cooling fan operates as a pump, and therefore the pump will try to increase the pump output flow rate until the pump output pressure reaches the pressure determined by the relief valve, and therefore the rate of intake flow of the operating fluid from the operating fluid tank to the pump will correspondingly increase, and the rotation rate of the hydraulic motor will increase, and there is a danger that the durability problems of the pump is seriously damaged.